All About Email Standards
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A different approach to email depends on a more inclusive model, where messages are exchanged not just between individual users of the same system. In this model, messages are actually transferred from a source system to a mail-forwarding system, which in turn delivers the messages to their destinations. These programs provide front ends for reading messages, and they are able to send and receive messages through the agency of message transfer agents (MTAs).
MTAs have links with other MTAs and are able to forward messages through the network cloud shown in Figure 1.2. One might think that the aggregation of all linked MTAs can be viewed as the functional equivalent of a centralized email repository, but this is not the case. The MTAs don’t necessarily retain messages, but may relay the messages only when UAs request them.
The MTAs make it possible for messages to be transmitted across systems, and open standards make it possible for those messages to be properly interpreted by recipient systems no matter what type of system was used to originate the messages. Finally, MTAs forward messages but do not affect their contents (if they’re working right, anyway).